Title

Location

San Diego, California

Session Start Date

3-26-2001

Session End Date

3-31-2001

Abstract

The paper presents a critical assessment of the currently prevailing view of structural engineers, as expressed in seismic codes, that the role of SSI is always beneficial for the design seismic forces developing in a structure. Using recorded strong ground motions and theoretical analyses it is shown that, in certain seismic and soil environments, an increase due to SSI in the fundamental period of a moderately flexible structure may have a detrimental effect on seismic demand, contrary to the conclusion drawn on the basis of idealized (“average”) code spectra. Using a simple 2-dof system and a number of actual ground motions as excitation, it is also shown that indiscriminate use of presently popular “geometric” ductility relations may lead to erroneous conclusions in the prediction of seismic performance of flexibly-supported structures. A significant case history, referring to the failure of the 630-m Fukae bridge section of the Hanshin Expressway Route 3 in Kobe (1995), further supports the main findings of the paper, by showing that soil-structure interaction may have played a decisive even if subtle role in that failure.

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The paper presents a critical assessment of the currently prevailing view of structural engineers, as expressed in seismic codes, that the role of SSI is always beneficial for the design seismic forces developing in a structure. Using recorded strong ground motions and theoretical analyses it is shown that, in certain seismic and soil environments, an increase due to SSI in the fundamental period of a moderately flexible structure may have a detrimental effect on seismic demand, contrary to the conclusion drawn on the basis of idealized (“average”) code spectra. Using a simple 2-dof system and a number of actual ground motions as excitation, it is also shown that indiscriminate use of presently popular “geometric” ductility relations may lead to erroneous conclusions in the prediction of seismic performance of flexibly-supported structures. A significant case history, referring to the failure of the 630-m Fukae bridge section of the Hanshin Expressway Route 3 in Kobe (1995), further supports the main findings of the paper, by showing that soil-structure interaction may have played a decisive even if subtle role in that failure.